Python Threading Semaphore信號量（子線程的數量）

原因：主要用在保護有限的資源。
假設當前數據庫支持最大連接數爲3，將信號量初始值設爲3，那麼同時最大可以有三個線程連接數據庫，其他線程若再想連接數據庫，則只有等待，直到某一個線程釋放數據庫連接。

注意：輸出結果是每兩秒打印的，跟數據庫連接操作差的遠呢

import logging
import threading
import time
import random
from threading import Semaphore

logging.basicConfig(
    level=logging.INFO,
    format="%(asctime)s [*] %(message)s"
)
threads = []
lock_sm = Semaphore(3)


class connectdb(threading.Thread):
    def run (self):
        while True:
            lock_sm.acquire()
            logging.info(f"{self.name} connecting to db... ")
            logging.info(f"{self.name} released db...")
            time.sleep(2)
            lock_sm.release()

if __name__ == '__main__':
    for i in range(5):
        threads.append(connectdb())
    
    # 從五個線程取出三個
    random_threads = random.sample(threads, 3)
    
    # 阻塞啓動線程---------------------------
    for t in random_threads:
        t.start()
    for t in random_threads:
        t.join()

輸出：

2019-10-02 17:31:38,851 [*] Thread-2 connecting to db... 
2019-10-02 17:31:38,851 [*] Thread-2 released db...
2019-10-02 17:31:38,851 [*] Thread-4 connecting to db... 
2019-10-02 17:31:38,851 [*] Thread-4 released db...
2019-10-02 17:31:38,851 [*] Thread-3 connecting to db... 
2019-10-02 17:31:38,851 [*] Thread-3 released db...
2019-10-02 17:31:40,851 [*] Thread-2 connecting to db... 
2019-10-02 17:31:40,851 [*] Thread-2 released db...
2019-10-02 17:31:40,852 [*] Thread-3 connecting to db... 
2019-10-02 17:31:40,852 [*] Thread-3 released db...
2019-10-02 17:31:40,852 [*] Thread-4 connecting to db... 
2019-10-02 17:31:40,854 [*] Thread-4 released db...
2019-10-02 17:31:42,853 [*] Thread-2 connecting to db... 
2019-10-02 17:31:42,853 [*] Thread-2 released db...
2019-10-02 17:31:42,854 [*] Thread-3 connecting to db... 
2019-10-02 17:31:42,854 [*] Thread-3 released db...
2019-10-02 17:31:42,855 [*] Thread-4 connecting to db... 
2019-10-02 17:31:42,855 [*] Thread-4 released db...
2019-10-02 17:31:44,854 [*] Thread-2 connecting to db... 
2019-10-02 17:31:44,854 [*] Thread-2 released db...
2019-10-02 17:31:44,855 [*] Thread-3 connecting to db... 
2019-10-02 17:31:44,855 [*] Thread-3 released db...
2019-10-02 17:31:44,856 [*] Thread-4 connecting to db... 
2019-10-02 17:31:44,856 [*] Thread-4 released db...